Slag is a common by-product of metal refining. All steelmaking processes including open-hearth, basic oxygen, electric arc furnace, argon-oxygen decarburization and other processes produce slag in their respective refining processes. Other metallic refining processes such as aluminum smelting, copper smelting and blast furnace processes produce slag. Metal refining is a means of separating a particular desired metal from the undesirable or waste product (in the form of slag.)
One of the inherent drawbacks in refining is that a certain amount of metal values being refined are oxidized in the refining process and precipitate out of the molten bath into the slag. Consequently, there are recoverable metals values, or "metallics", in most metal refining slags. Metal refiners have the option of recovering these metallics and they may then be used as part of an overall total metallic charge for a particular refining process.
There are many means of collecting slag. Open hearth furnaces are known for their flush holes whereby the center door of the furnace is the flush door. A hole in the floor allows slag seeping out of the furnace to pour directly into a pit below. After each heat, this pit is cleared of slag by being plowed or bucketed (via overhead crane bucket). The slag is then loaded into slag pots via mechanical means and then transported to a slag preparation site. Similarly, aluminum and copper smelting operations are noted for their slag dross.
Basic oxygen furnaces pour slag directly into a slag pot. Slag sticking to slag pots has long been a hindrance to steelmakers. Through the years, a common practice has developed to prepare a slag pot to receive slag by dumping sand or mill dirt or both into the bottom of each pot so as to deoxidize the slag as it enters the pot and thereby keep it from foaming. This also prevents the slag from sticking to the pot's lower quadrant. With the pot's shape being rather conical, the resulting shape of the slag when solidified is a frustoconical or elliptical solid body or "button".
Although slag pot designs vary, their shapes are basically similar. Electric furnace slag pots look much the same as basic oxygen slag pots. Argon-oxygen decarburization slag pots (depending on the size of the furnace) are generally smaller than most basic oxygen furnace slag pots. Hence, many argon-oxygen decarburization facilities spray a silicon coating on the walls of their slag pots rather than put sand in the bottom. The resulting shape of AOD slag is conical rather than that of a button. Aluminum and copper slags are skimmed from the bath and placed in a dross bucket. For the sake of simplicity, slag is in referred to in the forthcoming description of the present invention as being in the shape of a button.
Regardless of the shape of the solidified slag, there generally are a significant amount of metallics in it. Metal refiners wish to recover these metallics since they have already paid for them. For example, It is reasonable to recycle iron bearing slag and recharge it after it has been prepared.
After slag has been loaded into a slag pot, it is usually transported outside of the furnace building to a slag preparation area. This area is commonly referred to as the "slag pits". The slag buttons are transported to the slag pits and dumped. In the case of furnace slag, the slag is crushed, screened and magnetically separated according to its metallic content. Slag high in metallic content is charged back to the steelmaking furnace for iron recovery. Processed slag which does not meet the mills' criteria for contained iron is subsequently sold as construction aggregate. Buttons are frequently too thick to be broken with a drop ball. In order to prepare this large mass of ladle remains for the crusher, the button must be sectioned by a fuel gas torch while still warm. Buttons fresh out of the mill are extremely hot and usually molten in the center. Most slag burning is preferred to be done on buttons that are still warm since it requires less fuel to cut the button into sections. A typical button may range in thickness from four to eight feet and weigh up to 40 tons.
This setup is characteristic of many slag preparation operations. In addition to steel buttons, revert scrap as billets or finished mill product are also burned with oxygen and fuel gas torches. The scrap is torch cut into appropriate lengths, usually two to four feet long, for charging in the electric arc or basic oxygen furnace. Large mill torches, such as the Victor HC-1500C Scrap Torch, utilize natural gas or propane as fuel gas. Oxygen is subsequently introduced at high flowrates to cut or oxidize the metal by self-sustaining exothermic reactions. Oxygen flowrates from 50 to 80 standard cubic feet per minute at 75 to 150 pounds per square inch (gauge) are common for mill torches.
The inherent problem associated with these cutting operations of slag or scrap is the generation of metallic oxide fumes. These process fumes are large billowing clouds of oxide vapors typically orange and brown in color. Not only are the fumes uncomfortable for the torch operator, but they present an environmental problem for mill sites. These operations are performed outdoors where wind creates additional problems by dispersing the oxide clouds throughout neighboring premises and depositing the oxides on cars and buildings, or sometimes, into populated areas. This raises concerns about the environmental impact of oxide fumes from lancing or cutting operations as a potential pollutant.
It is believed that previous efforts directed to decreasing oxide fumes have been directed to the use of carbon dioxide in the oxygen feed of an oxygen-acetylene cutting torch where a fuel gas--acetylene--is used in cutting scrap. This is unsatisfactory for a number of reasons. This approach requires that a fuel gas be used along with its attendant cost and handling complexity. Further it necessitates that not only must the appropriate fuel gas-oxygen mixture be preserved to support cutting combustion but that somehow the carbon dioxide-oxygen mixture also must support that cutting combustion while attempting to decrease oxide fumes.